A 6.7 GHz Methanol Maser Survey at High Galactic Latitudes
Kai Yang, Xi Chen, Zhi-Qiang Shen, Xiao-Qiong Li, Jun-Zhi Wang, Dong-Rong Jiang, Juan Li, Jian Dong, Ya-Jun Wu, Hai-Hua Qiao, Zhiyuan Ren
AA 6.7 GHz Methanol Maser Survey at High Galactic Latitudes
Kai Yang , , Xi Chen , , , Zhi-Qiang Shen , , Xiao-Qiong Li , , Jun-Zhi Wang , ,Dong-Rong Jiang , , Juan Li , , Jian Dong , , Ya-Jun Wu , , Hai-Hua Qiao , , ZhiyuanRen , ABSTRACT
We performed a systematic 6.7 GHz Class II methanol maser survey using theShanghai Tianma Radio Telescope toward targets selected from the all-sky
Wide-Field Infrared Survey Explorer (WISE) point catalog. In this paper, we reportthe results from the survey of those at high Galactic latitudes, i.e. | b | > ◦ . Of1473 selected WISE point sources at high latitude, 17 point positions that wereactually associated with 12 sources were detected with maser emission, reflect-ing the rarity (1 − WISE images show that almost all of these masers are located inthe positions of the bright
WISE point sources. Compared with the methanolmasers at the Galactic plane, these high-latitude methanol masers provide goodtracers for investigating the physics and kinematics around massive young stellarobjects, because they are believed to be less affected by the surrounding clusterenvironment.
Subject headings: masers — stars: formation — ISM: molecules — radio lines:ISM Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030,China; [email protected], [email protected], [email protected]. University of Chinese Academy of Sciences, 19A Yuquanlu, Beijing 100049, China. Center for Astrophysics, Guangzhou University, Guangzhou 510006, China; [email protected]. Key Laboratory of Radio Astronomy, Chinese Academy of Sciences, China. National Astronomical Observatories, Chinese Academy of Sciences, A20 Datun Road, Chaoyang Dis-trict, Beijing 100012, China. a r X i v : . [ a s t r o - ph . GA ] A p r
1. Introduction
Methanol masers are very useful tools for investigating physics and kinematics in massivestar forming regions. Methanol masers have been divided into two classes (Class I and ClassII, Batrla et al. 1987). Voronkov et al. (2010, 2014) concluded that Class I methanol masersare pumped by collision excitation in mildly shocked molecular gas and reside on scales of ∼ (cid:48)(cid:48) , Caswell et al. 2010).The 6.7 GHz transition is an extensively studied Class II methanol maser and is one ofthe strongest masers known in our universe (Menten et al. 1988a,b; Norris et al. 1988; Menten1991). Unlike other maser species (OH and H O masers associated with both star formingregions and evolved stars), the 6.7 GHz methanol maser is only associated with massive starforming regions (Minier et al. 2003; Xu et al. 2008). A large number of observations suggestthat 6.7 GHz methanol maser is a powerful tool for studying massive star forming regions(e.g. Ellingsen 2006).In the past few decades, there were several 6.7 GHz methanol maser surveys in ourGalaxy, including targeted surveys (e.g. Menten 1991; MacLeod et al. 1992; Caswell et al.1995; Caswell 1996; Ellingsen et al. 1996; van der Walt et al. 1996; Walsh et al. 1997; Ellingsen2007) and unbiased surveys (e.g. Green et al. 2009, 2010, 2012). Targeted surveys mainlyobserved infrared sources or star forming regions associated with known tracers such as H Oor OH masers (e.g. Menten 1991), while unbiased surveys are performed toward specificareas. The Methanol Multibeam (MMB) Survey conducted with the Parkes telescope is anunbiased survey of 6.7 GHz methanol maser in a relatively wide region of the Galactic plane(186 ◦ ≤ l ≤ ◦ , | b | ≤ ◦ , Green et al. 2009). A catalog of about 519 6.7 GHz methanolmasers is compiled by Pestalozzi et al. (2005), and the MMB survey has detected 954 sources(Caswell et al. 2010; Green et al. 2010; Caswell et al. 2011; Green et al. 2012; Breen et al.2015). Up to now, about 1000 Class II methanol maser sources have been detected in ourGalaxy.Since star forming regions are considered to be located mainly in the Galactic plane,those 6.7 GHz methanol maser surveys in our Galaxy were conducted in the regions at lowGalactic latitudes ( | b | < ◦ ). In the catalog of Pestalozzi et al. (2005), only 14 sourcesare at high Galactic latitudes, suggesting the decrease of methanol maser sources towardhigh latitude. The MMB survey was also conducted toward the Galactic plane with | b | ≤ ◦ . Further systematic survey toward high-latitude regions are necessary to further checkwhether or not methanol masers are rare at high latitudes. 3 –Many previous interferometric observations (including connected element interferom-eters and VLBI) have revealed that some Class II methanol maser spots are distributedpredominately in lines or arcs with a near-monotonic velocity gradient along the structure(Norris et al. 1998; Phillips et al. 1998; Bartkiewicz et al. 2009, 2014, 2016; Fujisawa et al.2014; Sugiyama et al. 2016). Bartkiewicz et al. (2009) reported that about one-third of theirsample sources show a new class of “ring-like” methanol maser spot distribution. The newmorphology suggests that methanol maser emission is produced by shocked material associ-ated with a disk or a torus. However, some other observations also show that the methanolmaser spot distribution has other morphologies, such as complex and pair morphologies(Bartkiewicz et al. 2014, 2016). The complex methanol morphologies may be formed underthe complicated cluster or neighbor environment around the massive star forming regionswhere the methanol masers locate. Therefore, a relatively clean environment with less clus-tering is helpful to better investigate the origin of 6.7 GHz methanol masers. The 6.7 GHzmethanol maser sources at high-latitude ( | b | > ◦ ) regions would provide potential candi-dates for such study, because they have quite a simple cluster environment compared to starforming regions at the Galactic plane.Several infrared surveys toward specific regions have been carried out, such as the Galac-tic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) infrared survey (Benjaminet al. 2003) of the Galactic plane (-65 ◦ ≤ l ≤ ◦ , | b | < ◦ ) by Spitzer and the survey carriedout by the Midcourse Space Experiment (MSX) satellite ( | b | < ◦ ). The all-sky Wide-FieldInfrared Survey Explorer ( WISE ) catalogue covers the entire sky (Wright et al. 2010), andit is an up-to-date database. It observes at four mid-infrared bands: 3.4 µ m, 4.6 µ m, 12 µ mand 22 µ m, with resolutions of 6 . (cid:48)(cid:48) , 6 . (cid:48)(cid:48) , 6 . (cid:48)(cid:48) , 12 (cid:48)(cid:48) , respectively. These all make WISE agood database for the further methanol maser survey. In this paper, we report a survey of6.7 GHz methanol masers toward a sample of
WISE -selected sources at high-latitude ( | b | > ◦ ) regions. Sections 2 and 3, describe the sample selection and observations. We presentthe results in Section 4, followed by discussions in Section 5 and a summary in Section 6.
2. Sample Selection
We used the MMB survey catalog as a starting sample to select the
WISE point sourcecandidates, which harbor the 6.7 GHz methanol maser. The MMB survey catalog has aposition accuracy of 0 . (cid:48)(cid:48) by ATCA (Green et al. 2009) and the astrometric precision of the WISE point sources is better than 0 . (cid:48)(cid:48) for high signal-to-noise ratio (S/N) sources andthere is an additional error of about FWHM/(2 S/N) for S/N <
20 sources (Wright et al.2010), the FWHM represents the full width at half maximum of the point − spread function. 4 –The positions of the maser sources in the MMB catalog were determined at an accuracy ofsub-arcsecond. The MMB catalog includes a total of 684 masers in the range of longitude186 ◦ ≤ l ≤ ◦ and latitude | b | < ◦ . We cross-matched the MMB catalog sources with theAllWISE catalog sources using 7 (cid:48)(cid:48) radius as the criteria. The AllWISE source catalog hasaccurate positions and four-band fluxes for ∼
750 million sources extracted from its all-skysurveys (Mainzer et al. 2011). We select 7 (cid:48)(cid:48) radius as a matching criterion based on thespatial resolution of
WISE at shorter wavelengths and the possible spatial distribution of6.7 GHz methanol masers (usually less than 1 (cid:48)(cid:48) ; e.g. Caswell 2009). As a result, we foundthat a total of 502 methanol maser sources in the MMB catalog have
WISE counterparts.There are five maser sources, each of which is matched with two
WISE point sources. Forthem, the angular separation of the two
WISE point sources associated with each of them isusually larger than 3 (cid:48)(cid:48) and we kept the closest
WISE point source only. Furthermore, we onlyconsidered those
WISE point sources (473 in total) with all the four-band flux measurementsavailable.On the basis of the magnitude and color-color analysis for the selected 473
WISE sourceswith 6.7 GHz MMB methanol maser associations, we further try to build up the criteria forthe candidate
WISE sources that may have the 6.7 GHz methanol masers on the full sky.Fig. 1 shows the diagrams of the magnitudes of the
WISE four bands (3.4, 4.6, 12, and 22 µ m) and the [3.4]–[4.6] versus [12]–[22] color of the selected sources. As a comparison, thecorresponding distributions of those within a 1 ◦ diameter of the two locations of l = 90 ◦ , b = 70 ◦ (denoted G90+70), and l = 0 ◦ , b = 0 ◦ (denoted G00+00) are overlaid on Fig. 1.The comparison fields contain 13,627 and 18,939 WISE sources for G90+70 and G00+00,respectively. Firstly, we select the
WISE sources with the magnitude criteria: [3 . < . < < < . µ m data meet these criteria, and only one source satisfies the12 and 22 µ m criteria. Second, we further select the sample with the WISE color criteria:[3 . − [4 . >
2, and [12] − [22] >
2. Toward this defined color region, we found that themajority of the methanol masers (330/455 = 73%) fall in, whereas only nine
WISE sourcesfrom the comparison field of G90+70 locate (Fig. 1 (c)). Fig. 2 shows the histogram of thedistribution of the angular separations between the 330
WISE sources and their associatedMMB masers. We found that most of them (288/330 = 87%) are associated with the MMBmasers within an angular scale of less than 3 (cid:48)(cid:48) . Even for the comparison field of G00+00,where the background of the Galactic center is very complicated, there is still only a verysmall fraction (111/18,939=0.6%) of the
WISE sources in this color region (Fig. 1 (d)). Byapplying the above criteria to the full sky, we found a total of ∼ WISE × ) of the ALLWISE sources. Notably, inorder to detect as many methanol masers as possible, we select ALLWISE sources satisfyingthe above criteria without considering the photometric accuracy factor of the WISE sources.We also checked the
WISE sources with low S/Ns, e.g. S/N <
3, associated with the MMBmasers, and found that most of these sources also meet our selection criteria.From these candidate
WISE sources, we further select a total of 3,348
WISE sources asour targeted sample of the 6.7 GHz class II methanol masers to be observed by the ShanghaiTianMa radio telescope (TMRT). This sample excluded the sources in the Parkes MMBsurvey and those with a declination of less than − ◦ . In this targeted sample, 1473 sourceslocate at the high Galactic latitude region with | b | > ◦ (Table 1).
3. Observations
The observations were performed between 2015 September and 2016 July with theTMRT in Shanghai, China. This telescope is a new fully steerable radio telescope witha diameter of 65m. We used a cryogenically cooled receiver covering the frequency rangeof 4 − → A + transition line (6.6685192 GHz) was covered with a spectral window of a bandwidthof 23.4 MHz. The window has 16,384 channels and the spectral resolution is 1.431 kHz,corresponding to a velocity resolution of ∼ − . The temperature of this systemis about 20 −
30 K. The aperture efficiency of the TMRT is ∼
55% and the correspondingsensitivity is 1.5 Jy K − . The beam size is ∼ (cid:48) (HPBW) at the frequency of 6.7 GHz. Theflux densities for the sources may have an uncertainty of less than 20%, estimated from theobserved variation of the calibrators.A position-switching mode was used in our observations. Each source was observed withtwo ON − OFF cycles. Procedures of both the ON position and the OFF position in eachcycle take ∼ ◦ , − ◦ ) from theON position in (R.A., decl.).We used the GILDAS/CLASS package to process the observed spectral data. We fittedand subtracted the linear baseline of the spectrum. The root-mean-square (rms) noise levelis about 20 −
40 mJy.For the detected methanol maser sources (12 in total, see below), we used the on-the-fly(OTF) mode (Dong et al. 2016) to map a large region (usually 5 (cid:48) ) to obtain a position of 6 –the methanol maser. The TMRT under the OTF mode moved in a given region along thedirection of R.A. first, then the direction of decl. Only one spectral window is used to coverthe methanol maser.
4. Results4.1. Detections
We detected 6.7 GHz methanol masers toward positions of 17
WISE point sources,within actually 12 sources. The properties of these detected methanol maser sources arelisted in Table 2. Three sources are newly detected and Fig. 3 shows their spectra. Weconfirmed that another nine methanol maser sources detected by the TMRT observationswere already reported after taking their similar positions (within 1 (cid:48) ) and spectral velocityranges into account. Their spectra are shown in Fig. 4. Below are short descriptions of these12 sources.
G97.527+3.184 . This source has three peaks at − − − − ,within a velocity range of − − to − − . G110.196+2.476 . This weak source has a peak flux density of 0.5 Jy at − − . G137.068+3.002 . This source has a peak flux density of 1.6 Jy at − − .Compared to previously known sources, all the three new sources show relatively weakmaser intensity. G110.196+2.476 and G137.068+3.002 have a simple spectral feature.G97.527+3.184 has also been studied by Hachisuka et al. (2015) through the 22 GHzH O maser and it is located in the Outer Arm. So, this source may provide a good candidateto study the massive star formation that occurred in the regions at the edge of our Galaxy.Using the multi-band infrared data, including the
WISE and
Herschel , we can estimatethe related physical parameters for the newly detected sources. However, in the three sources,only one source, G110.196+2.476, is covered by
Herschel .We estimated the dust temperature of G110.196+2.476 from its spectral energy distri-butions (SED) in the
WISE µ m, Herschel
PACS 70 and 160 µ m, and SPIRE 250, 350, 7 –and 500 µ m. The SED is fitted using a gray-body emission model (Hildebrand 1983) S ν = κ ν B ν ( T d )Ω µm H N tot /g = κ ν B ν ( T d ) M core gD , (1)wherein S ν is the flux density at the frequency ν . Ω is the solid angle of the core or theselected area. B ν ( T d ) is the Planck function of the dust temperature T d . µ = 2 .
33 is themean molecular weight (Myers & Benson 1983), m H is the mass of the hydrogen atom, g = 100 is the gas-to-dust mass ratio, N tot is the column density of the core, D = 5 . κ ν is the dust opacity and is expected to vary with the frequencyin the form κ ν = κ ( ν/ β , with the reference value of κ = 0 . g − ,as adopted from dust model for the grains with coagulation for 10 years with accreted icemantles at a density of 10 (Ossenkopf & Henning 1994).The continuum emission of the core has a compact and isolated shape throughout allthe wavebands. The flux density can be reproduced by a single temperature component,as shown in Fig. 5. The derived physical parameters are M core = 14 M (cid:12) , T d = 49 K and β = 1 . G16.872 − − . The methanol maser emission detected fromthese two positions actually should be from a common position close to G16.872 − − − G17.021 − . This source was detected by Szymczak et al. (2000), named 18277 − − and 8.9 Jy at 20.6km s − in Szymczak et al. (2000) and our observations, respectively. G78.122+3.633 . This source was detected by MacLeod et al. (1992), Slysh et al.(1999), Szymczak et al. (2000), and Minier et al. (2000, 2001), named 20126+4104. TMRTmeasured a peak flux density of 39.9 Jy at − − , similar to 38 Jy at − − inSzymczak et al. (2000). G108.184+5.518 . It was detected by MacLeod et al. (1998), Slysh et al. (1999), andSzymczak et al. (2000). This source has a peak flux density of 49.2 Jy at − − by 8 –the TMRT. In Szymczak et al. (2000), it (named 22272+6358) had a peak flux density of69.4 Jy at − − . G109.839+2.134, G109.868+2.119 . The spectra of these two separate sourcesshow that most maser emission is from G109.868+2.119, which has a very strong peakflux density of 766.7 Jy at − − . G109.868+2.119 was detected by Slysh et al.(1999) and Szymczak et al. (2000). Szymczak et al. (2000) reported G109.868+2.119 (named22543+6145) with a feature of peak flux density of 815 Jy. G123.035 − − . The maser emission detected toward thesetwo positions are from the same source: G123.050 − − G173.482+2.446 . This 6.7 GHz methanol maser site was detected by Menten (1991),Szymczak et al. (2000) and Minier et al. (2000), named S231. In Szymczak et al. (2000), itshowed a much stronger flux density (a peak flux density of 208 Jy) than our detection (41.5Jy), with a quite different spectrum.
G173.596+2.823, G173.617+2.883 . The spectra at both positions clearly showthat most emission is from G173.617+2.883, with a velocity range of − − to − − . G173.617+2.883 was detected by Szymczak et al. (2000), and was named05382+3527. G213.706 − − . These two positions have features at thesame velocity range and the emission is mostly from G213.706 − − − − . However, we found two peaks with peak flux densities of ∼
120 Jy at both10.5 and 12.7 km s − . This source, named Mon R2, was also detected in Menten (1991),Caswell et al. (1995), Walsh et al. (1997, 1998), and Minier et al. (2001). The center coordinates and the side length of the square regions chosen for the OTFobservations toward these 12 sources are listed in Table 3. Fig. 6 shows the velocity-integrated intensity map from the OTF observation and the 6.7 GHz methanol maser spectrain the fitting center toward G97.527+3.184, the peak positions in each region can be foundin Table 3. Considering the pointing error of the telescope (about 10 (cid:48)(cid:48) ) and the fitting error 9 –from the OTF mapping observation (typically 10 (cid:48)(cid:48) ), we can obtain a positional accuracy ata level of better than 20 (cid:48)(cid:48) with the TMRT OTF observations.Minier et al. (2000, 2001) and Hu et al. (2016) have studied eight of these sourcesthrough interferometric observations with the European VLBI Network (EVN), the VeryLong Baseline Array, and the Very Large Array (VLA). For comparison, the interferometricpositions of these eight sources are also listed in Table 3, along with their position dif-ference from our OTF measurements. The positions of the six sources (G17.021 − − (cid:48)(cid:48) ∼ (cid:48)(cid:48) , with regardto the positions determined from the interferometric observations. This is consistent with apositional uncertainty of less than 20 (cid:48)(cid:48) from the OTF observations. For G16.872 − − (cid:48)(cid:48) and10 (cid:48)(cid:48) , respectively.
5. Discussions5.1. Rarity of high-latitude methanol masers
In total, 17 methanol maser sources at high latitudes, including known sources from thePestalozzi et al. (2005) catalog and those first reported in this work, have been detected todate. The distribution of 6.7 GHz methanol maser sources at high latitude is shown in Fig.7. It can be seen that about three-fourths (13 of 17 sources) are located at Galactic latitudesof 2 ◦ ∼ ◦ and − ◦ ∼ − ◦ . According to statistics (Pestalozzi et al. 2005; Caswell et al.2010; Green et al. 2010; Caswell et al. 2011; Green et al. 2012; Breen et al. 2015), there areabout 1000 Class II methanol maser sources detected in our Galaxy, suggesting a ∼ . ◦ , consistent with the rarity ( ∼ . ◦ ≤ l ≤ ◦ and | b | > ◦ . Assuming a uniform distribution of methanol masers along the galactic longitude,then a total number of 20 is estimated to be detected in the whole high-latitude region (0 ◦ ≤ l ≤ ◦ , | b | > ◦ ) toward the WISE -selected sample. Because the detection rate is estimatedto ∼
73% (see § | b | > ◦ . Thus, our survey strongly suggests that the 6.7 GHz methanol maser is really rare 10 –in the high-latitude region. As described in Section 4.1.2, 9 of 12 TMRT detected methanol maser sources werepreviously observed by Szymczak et al. (2000) with the 32m Toru´n radio telescope withan HPBW of 5 . (cid:48) at 6.7 GHz. These sources are not accurately pointed from the samecoordinate position, which may cause differencec in flux density. Our discussion will mainlyfocus on the variations of their spectral profiles. We also list the source names in parenthesisfrom Szymczak et al. (2000) at the beginning of the description of each source. G16.872 − − . The spectra of both G16.872 − − − of G16.872 − − of 18265 − − . Such an offset may not be significant compared to the reported error of ± − in the absolute radial velocity. G17.021 − − . From the spectra of this source and 18277 − − and anobscure feature at about 20.6 km s − . However, our spectra show features at 19.4, 20.6, 21.7,23.3, and 24.4 km s − . G78.122+3.633 (20126+4104) . The peak flux densities of G78.122+3.633 and20126+4104 are 40.9 Jy and 38 Jy, respectively. Except for a stable feature at − − ,20126+4104 did not show any other features seen in G78.122+3.633 at − − − − − . G108.184+5.518 (22272+6358) . The peak flux densities of G108.184+5.518 and22272+6358 are 49.2 and 91 Jy, respectively. Compared to Szymczak et al. (2000), featuresat about − − and −
10 km s − remain unchanged in our observation, whereas anew feature at about − − arose in our observation. G109.868+2.119 (22543+6145) . We can see that the spectra of G109.868+2.119and 22543+6145 have similar features and flux densities. It seems that this source has notchanged a lot.
G123.050 − . From the spectra of G123.050 − − − and − − have similar flux density ratios, but become stronger now. G173.482+2.446 (05358+3543) . The peak flux densities of G173.482+2.446 and05358+3543 are 41.5 and 256 Jy, respectively. We can clearly see features at − − − − in both observations. Moreover, there is a new feature at about − − . In addition, the peak flux densities have decreased a lot. G173.617+2.883 (05382+3547) . The peak flux density has changed from 7.5 Jyin 05382+3547 to 4.5 Jy in G173.617+2.883 and a new feature arose at − − . G213.706 − − . The peak flux densities of G213.706 − − − and12.5 km s − disappeared and the new feature at about 12.7 km s − showed up with a fluxdensity as strong as that of the feature at about 10.5 km s − .The 3 σ noise level was typically 1.5 − The infrared three-color images of these 12 TMRT detected methanol maser sourcesare shown in Fig. 8 with the fitting centers of methanol emission determined from theTMRT OTF observations marked. Two sources (G17.021 − WISE infrared image. The positions of three sources(G108.184+5.518, G110.196+2.476, and G173.482+2.446), determined from the TMRT OTFobservations are within 20 (cid:48)(cid:48) from the nearby bright infrared sources. All of the remainingseven methanol maser sources locate in the bright
WISE sources. Considering the 20 (cid:48)(cid:48) positional accuracy of TMRT OTF observations (see § WISE sources.Furthermore, with more accurate interferometric positions for eight sources (see § WISE sources, with an exception ofG17.021 − − − WISE sources, which were targeted in our survey. Therefore, we could also refine a methanol masersource at a positional accuracy of about 10 (cid:48)(cid:48) ∼ (cid:48)(cid:48) , by solely matching to the WISE sourcepositions. This is especially important for those maser sources without any high accuracyposition measurements, such as those from surveys toward a sample selected from
WISE sources at a low-latitude region.
The 6.7 GHz methanol maser distributions toward eight sources have been studied withinterferometric observations (see § − − − − − shaped distribution of the masers may be associated with jet/outflow or disk/torusstructures (e.g. Bartkiewicz et al. 2016) from an isolated massive young stellar objects.However, the complex structure of the masers should be affected by the neighborhoods in acomplicated cluster environment in the massive star forming regions.Of the 63 low-latitude sources with the EVN imaging of 6.7 GHz methanol masers(Bartkiewicz et al. 2009, 2014, 2016), 1 source shows simple morphology, 11 sources showring morphology, 13 sources show linear morphology, 5 sources show arched morphology, 29sources show complex morphology, and 4 sources show pair morphology. This proportionis quite different from our high-latitude ones, especially for the complex morphology group:only one in 8 sources from our sample, compared to about half (29/63) in Bartkiewicz et al.(2009, 2014, 2016) samples. This is likely to be due to the fact that the sources at the lowlatitudes usually have more complicated cluster environments. The physical and kinematiccontributions from the neighborhoods around the host of methanol masers would complicatethe morphology of methanol masers. This makes it difficult to use a methanol maser as auseful probe to the physics and kinematics that are solely associated with its host. From thispoint of view, the detected methanol maser sources at high latitudes from our observationswould serve as better candidates for studying the physics and kinematics during the massivestar formation in our further work using VLA observations. 13 –
6. Summary
Using the newly built TMRT in Shanghai, we undertook a systematic survey of 6.7GHz methanol masers toward high-latitude ( | b | > ◦ ) targets in our Galaxy. The targetsample contains 1473 high-latitude sources selected from the WISE source catalog. Wedetected 12 methanol masers with 3 new detections, suggesting that the 6.7 GHz methanolmasers are really rare at the high-latitude region. The peak flux densities of the detectedsources are from 0.9 to 838.8 Jy, while new detections have peak flux densities in the rangeof 0.9 ∼ WISE point sources in the infrared images. The publishedinterferometric observations revealed that almost all of these high-latitude sources do nothave complex morphology in the methanol maser spot distribution, in contrast to those atlow latitudes. Therefore, high-latitude methanol masers might serve as an efficient tracer forstudying physics and kinematics of their hosts during the massive star formation.
Acknowledgements
We are thankful for the assistance from the operators of the TMRT during the obser-vations and for helpful comments that improved the manuscript. This work was supportedby the National Natural Science Foundation of China (11590780, 11590781, and 11590784),the Strategic Priority Research Program “The Emergence of Cosmological Structures” of theChinese Academy of Sciences (CAS), Grant No. XDB09000000, the Knowledge InnovationProgram of the Chinese Academy of Sciences (Grant No. KJCX1-YW-18), the Scientific Pro-gram of Shanghai Municipality (08DZ1160100), and Key Laboratory for Radio Astronomy,CAS.
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This preprint was prepared with the AAS L A TEX macros v5.2.
17 – T a b l e : S e l ec t e dS o u r ce C a t a l og N u m b e r l b R . A . D ec l. w ( . µ m ) w ( . µ m ) w ( µ m ) w ( µ m ) w − w w − w ( ◦ )( ◦ )( J )( J )( m ag )( m ag )( m ag )( m ag )( m ag )( m ag ) ( h m s )( ◦ (cid:48)(cid:48)(cid:48) ) ( )( )( )( )( )( )( )( )( )( )( ) . . : : . − : : . . . . − . . .
30 20 . . : : . − : : . . . . . . .
51 30 . − . : : . − : : . . . . . . .
58 40 . − . : : . − : : . . . . . . .
96 50 . . : : . − : : . . . . . . .
21 60 . . : : . − : : . . . . . . .
24 70 . − . : : . − : : . . . . . . .
04 80 . . : : . − : : . . . . . . .
44 91 . − . : : . − : : . . . . . . .
12 101 . . : : . − : : . . . . . . . ......
18 – T a b l e : P r o p e rt i e s o f t h e T M R T d e t ec t e d m e t h a n o l m a s e r s t o w a r d W I S E p o i n t s o u r ce s . N a m e R . A . D ec l. E p o c h ∆ V v p S p S i D i s t a n ce R e f . O t h e r N a m e s ( l, b )( J )( J ) yy / mm / dd ( k m s − )( k m s − )( J y )( J yk m s − )( k p c ) ( ◦ , ◦ )( h m s )( ◦ (cid:48)(cid:48)(cid:48) ) ( )( )( )( )( )( )( )( )( )( )( ) G . − . : : . − : : . . , . . . . . a C , W , S L − I R S . . . G . − . : : . − : : . . , . . . . C , W , S . . . G . − . : : . − : : . . , . . . . . a S L − I R S G . + . : : . + : : . − . , − . − . . . . a G , V , S , M , M I R A S + G . + . : : . + : : . − . , − . − . . . . b N I R A S + − . . . − . . . G . + . : : . + : : . − . , − . − . . . . a G , V , S L G . + . : : . + : : . − . , − . − . . . K , S , M G . + . : : . + : : . − . , − . − . . . . a K , S , M C e p A G . + . : : . + : : . − . , − . − . . . . c N G . − . : : . + : : . − . , − . − . . . S G . − . : : . + : : . − . , − . − . . . . a S N G C S G . + . : : . + : : . − . , − . − . . . . c N G . + . : : . + : : . − . , − . − . . . . a K , S , M S I R A S + G . + . : : . + : : . − . , − . − . . . S G . + . : : . + : : . − . , − . − . . . . d SS G . − . : : . − : : . . , . . . . . a K , C , W , S , M M o n R . . . G . − . : : . − : : . . , . . . . K , C , W , S , M . . .
19 – T a b l e : T h e O T F o b s e r v a t i o n s N a m e O T F P a r a m e t e r s P r o p e rt i e s o f t h e F i tt i n g C e n t e r s P r o p e rt i e s o f S o u r ce s b y I n t e r f e r o m e tr i c O b s e r v a t i o n D i ff e r e n ce R . A . D ec l. S i d e E p o c h R . A . D ec l. v p S p S i R . A . D ec l. R e f . ( l, b )( J )( J ) L e n g t h yy / mm / dd ( J )( J )( k m s − )( J y )( J yk m s − )( J )( J )( (cid:48)(cid:48) ) ( ◦ , ◦ )( h m s )( ◦ (cid:48)(cid:48)(cid:48) )( (cid:48) )( h m s )( ◦ (cid:48)(cid:48)(cid:48) )( h m s )( ◦ (cid:48)(cid:48)(cid:48) ) ( )( )( )( )( )( )( )( )( )( )( )( )( )( ) G . − . : : . − : : . : : . − : : . . . . : : . − : : . H . . . . G . − . : : . − : : . : : . − : : . . . . : : . − : : . H . G . + . : : . + : : . : : . + : : . − . . . : : . + : : . M , M , H . G . + . : : . + : : . : : . + : : . − . . . − . . . − . . . G . + . : : . + : : . : : . + : : . − . . . : : . + : : . H . G . + . : : . + : : . : : . + : : . − . . . : : . + : : . M , H . G . + . : : . + : : . : : . + : : . − . . . G . − . : : . + : : . : : . + : : . − . . . : : . + : : . H . G . + . : : . + : : . : : . + : : . − . . . G . + . : : . + : : . : : . + : : . − . . . : : . + : : . M , H . G . + . : : . + : : . : : . + : : . − . . . G . − . : : . − : : . : : . − : : . . . . : : . − : : . M , H . . . .
20 – [ ] - [ ][ ] - [ ] M agn i t ude ( m ) M agn i t ude ( . m ) a b c d [3.4] - [4.6][3.4] - [4.6] Magnitude (12 m) Magnitude (3.4 m)
Fig. 1.— Magnitudes and the [3.4]–[4.6] vs. [12]–[22] colors of the selected sources. The redpoints represent the 473
WISE sources associated with the 6.7 GHz methanol masers fromthe Parkes MMB Survey. The black points represent
WISE sources from two comparisonfields: G90+70 (a, b, c) and G00+00 (d). The defined box regions represent the magnitudesand color-color criteria for the
WISE -selected sources for the methanol survey in our work. 21 –
Angular separation (arcsec) N u m be r Fig. 2.— Histogram of the distribution of the angular separation between the 330
WISE sources and their associated MMB masers. Most of them (288 sources) have an angularseparation of less than 3 (cid:48)(cid:48) . 22 –Fig. 3.— Methanol spectra of three newly detected sources from the TMRT observations. 23 – F i g . . — T M R T s p ec tr ao f n i n e p r e v i o u s l y d e t ec t e d m e t h a n o l m a s e r s o u r ce s . F o rt h o s e s o u r ce s h a v i n g t h e s a m e m e t h a n o l o r i g i n , w e p l o tt h e i r . G H z m e t h a n o l s p ec tr a i n t h e s a m e p a n e l w i t hb l a c k a nd r e d li n e s . I n o r d e rt o c o m p a r e t h e w e a k s o u r ce s w i t h t h e s tr o n g s o u r ce s i n t h e s a m e s p o t , w e m u l t i p li e d t h e flu x d e n s i t i e s o f t h e w e a k s o u r ce s b y s e v e r a l t i m e s , w h i c h a r e w r i tt e n i n r e d w o r d s i n t h e p a n e l, s u c h a s G . − . x .
24 –
Wavelength F l u x d e n s i t y ( J y ) Fig. 5.— Spectral energy distribution of G110.196+2.476. The data points at wavelengthsof 12 and 22 µ m are detected by the WISE , and those at wavelengths of 70, 160, 250, 350,and 500 µ m are from the Herschel (http://irsa.ipac.caltech.edu). 25 – F i g . . — L e f t : t h e v e l o c i t y - i n t e g r a t e d i n t e n s i t y m a p o f G . + . f o rt h e . G H z m e t h a n o l m a s e r . W e d e r i v e d t h e v e l o c i t y i n t e n s i t y f r o m t h e s p ec tr ao f t h i ss o u r ce f r o m - . t o − . k m s − . T h e “o” r e p r e s e n t s t h e p o s i t i o n o f t h e W I S E s o u r ce i n t h e s i n g l e - p o i n t - s u r v e y , a nd t h e “ + ” r e p r e s e n t s t h e fi tt e dp e a k p o s i t i o n o f t h e m e t h a n o l e m i ss i o n f r o m t h e T M R T O T F o b s e r v a t i o n . R i g h t : s p ec tr a i n t h e fi tt i n g ce n t e r . ( T h ec o m p l e t e fi g u r e s o f t h e o t h e r d e t ec t e d s o u r ce s a r e a v a il a b l e o n li n e . )
26 – G a l a c t i c L o n g i t u d e ( d e g r ee s ) Galactic Latitude (degrees) F i g . . — G a l a c t i cc oo r d i n a t e s ’ d i s tr i bu t i o n o f t h e . G H z m e t h a n o l m a s e r s o u r ce s a t h i g h l a t i t ud e , | b | > ◦ . T h e r e d “ (cid:78) ” r e p r e s e n t s p r e v i o u s l y d e t ec t e d s o u r ce s ( P e s t a l o zz i e t a l. ) . T h e b l u e “ + ” r e p r e s e n t ss o u r ce s d e t ec t e d i n o u r w o r k .
27 –Fig. 8.— Infrared three-color images of the 12 detected methanol sources. The blue, green,and red colors represent 3.4, 4.6, and 12 µ m bands in WISE . The “ (cid:78) ” represents the fittedpeak position of methanol emissions by the TMRT OTF observation (Table 3), the “ (cid:4) ”represents the positions of